In a new video released by the Nigerian radical outfit, the insurgent chief has voiced support for the ongoing fight led by Sunni militant group Islamic State of Iraq and the Levant (Isis) in Iraq, against the Shiite-dominated Baghdad administration.

During the 16-minute video, which was released over the weekend and was first obtained by the AFP, Shekau also claimed responsibility for the bombings that rocked Abuja and Lagos recently.

"We were the ones who detonated the bomb in filthy Abuja. A bomb went off in Lagos. I ordered the [suicide-bomber] who went and detonated it. You [Nigerian authorities] said it was an incident. Well, if you hide it from people, you can't hide it from Allah," said Shekau.

Nigerian officials have contested the claim and insisted they were accidents.

In his remarks, the Islamist leader has also mocked the "Bring Back Our Girls" campaign, which has gained global prominence since the high school girls were abducted near three months ago.

Taking a dig at the Nigerian army, the militant figure derisively says in the video: "Bring Back Our Girls. Ooooh! Bring back our army."

Nigerian government officials have so far not commented on the contents of the video message but said they are examining them.

"We are studying the video. Our approach is to first conduct a thorough IT and forensic analysis of the video in order to establish its authenticity or otherwise. It is only after the investigation that we will be in a position to make an evidence-based stand," Force Police public relations officer Frank Mba told local daily Punch.